It is known that, generally, a roll mill comprises, inside a fixed stand, at least two working rolls delineating an air-gap for the product to run along a rolling plane. In order to maintain the space between the working rolls, which determines the reduction in thickness of the product, the said working rolls rest, generally, via rolls with larger diameter, so-called back-up or bearing rolls, on clamping means resting themselves on the stand.
A `quarto`-type roll mill, for instance, comprises two working rolls resting respectively on two back-up rolls. In a `sexto`-type roll mill, an intermediate roll is interposed between each working roll and the corresponding back-up roll. But other types of roll mills are also known.
The roll stand comprises normally two vertical roll standards connected by a crosspiece and on which are mounted clamping means resting, respectively, on each end of a roll with adjustable level, generally a back-up roll. Each roll is carried by a rotation shaft rotating, at both its ends, inside bearings each installed in a supporting block, called a `chock` and capable of sliding, along a clamping plane running through the axes of the working rolls, inside a window provided inside the corresponding roll standard of the stand.
All these arrangements are well known and do not call for any detailed description.
Until now, the rolls were clamped by screw-nut type mechanical devices, for instance, a screw resting, via an internal end, on the corresponding chock of the back-up roll and engaging into a fixed nut resting, in the opposite direction, on the roll standard, whereas the screw was driven into rotation in one direction or the other in order to grip or to release the rolls.
However, for several years, hydraulic clamping devices that comprise, on each roll standard, two elements sliding into one another, respectively a mobile element resting on the chock of the roll and a fixed element resting on the roll standard, have been used preferably.
Using hydraulic jacks enables, in particular, to facilitate the control of the rolling load and to regulate the thickness of the product rolled. To this end, it is necessary to provide rapid response of the system and, consequently, to know with accuracy the position of the roll that is adjustable in relation to the stand, from which the positions of the other rolls can be determined.
To this end, the applicant has already suggested, in the French patent no. 2.645.051, a roll position registration device comprising, in each roll standard, a measuring rod extending along the axis of the clamping jack between an internal end turned to the window and fixed on the mobile element of the jack and passing through the roll standard up to an external end connected to a position transducer. This rod must therefore go through the pressure chamber of the jack and, in this view, it has to pass through an insulating sleeve with one internal end fixed on the mobile element of the jack and an insulating section mounted to slide in relation to the fixed element, whereas a sliding seal is interposed in order to maintain the pressure inside the chamber of the jack without disturbing the displacements of the mobile element of the said jack.
The measuring rod goes through the tubular sleeve with a certain clearance in order to avoid any disturbance of the measurement, whereby the rod has no contact with any section of the roll mill, apart from the mobile element of the jack on which it is fixed.
Thanks to such an arrangement, it is possible to know at any time, with accuracy, the position with respect to the stand of the mobile element, of the jack and of the roll on which the clamping load is applied, in order to make the necessary corrections, for example, to take into account any yields and other deformations due to the application of the clamping load or to correct any thickness or surface evenness defects detected downstream.
Moreover, it must also be possible to move the jacks over a wide amplitude to suit various thicknesses of the product as well as variations in diameter of the rolls that are subject to quite a severe wear. These rolls must, besides, be replaced periodically and, in this view, they should be brought into a dismantling position for which they are spread apart from one another.
Manoeuvring the rolls can be made via auxiliary jacks, but it is also possible to use two-direction double-action clamping jacks. Normally, the body of the jack is fixed and rests on the roll standard, whereas the mobile piston 5 rests on the rolled to be adjusted and delineates two chambers, respectively a main chamber dedicated to the application of the clamping load and a ring chamber for restoring the jack into the opposite direction. However, this arrangement complicates the oil supply system of the jack, which is not symmetrical. Moreover, tightness in both chambers of the jack is not easy to provide, taking into account the necessary high pressures, and generates frictions that are detrimental to thickness regulation.
It is therefore easier to use single-action jacks that comprise a single chamber, whereas the application of the pressure determines the displacement of the piston only in the clamping direction. But it is necessary to provide means for restoring the piston into the opposite direction.
During the rolling operation, the restoring force is provided by the product itself. When the machine has stopped, conversely, only the auxiliary jacks are available, which are used normally for balancing the weight of the rolls and, if needed, for exerting bending to compensate for the deflection of the rolls. However, these balancing jacks act on the chocks of the rolls and can therefore not be used for maintaining the clamping jacks in raised position, during roll changing phases before proceeding to maintenance operations.
Therefore, when single-action jacks are used, they may be associated with locking devices fitted with clamping claws, actuated by auxiliary jacks, that maintain at least the jacks under their own weights during dismantling operations. Such devices, still, have not been designed to operate within the framework of a normal sequence and can be destroyed in case of mistakes since they, obviously, have not been designed to sustain the thrust of the clamping jacks. Such accidents cause stoppages for repairs that are extremely detrimental to production.
The invention allows to obviate such drawbacks while suggesting a particularly simple restoring device, applicable to single-action clamping jacks and that advantageously uses the existing arrangements, without complicating the roll stand any further.
Moreover, the invention is especially useful for retrofitting the existing stands.